In analog and mixed-signal communication circuits, it is often desired to measure the frequency spectrum of signals generated by integrated circuits, components, devices, etc. For example, an output signal of a PLL (phase locked-loop) may be analyzed to determine if the signal frequency is at a required value, or to analyze the phase noise of the output signal, or to determine if there is any crosstalk from noise sources with nearby frequencies.
Typically, frequency spectrum analysis is performed by bringing a signal of interest off the chip via a dedicated I/O (input/output) pin and measuring the frequency content using an external spectrum analyzer. For this purpose, the I/O pin must be connected to the circuit of interest via a high bandwidth output driver that does not distort the signal to be measured. However, there are many disadvantages associated with this conventional process.
For example, the conventional method imposes a limit on the number of internal signals that can be measured. In particular, in a case where there is an array of circuits, such as a parallel bus of data channels all operating at similar, but not identical, frequencies, a frequency spectrum measurement using the conventional method requires multiple connections and disconnections. Thus, the measurement is time-consuming and not suitable for rapid testing. Furthermore, the number of points which can be measured is limited by the number of I/O pins that can be dedicated for the purpose of frequency spectrum measurements.